There are three parts as mentioned already, and each section varies in point of view, with interesting uses of this literary element. In the first section, the omniscient narrator is employed, so the narration tells the reader all about the character and event, supposedly; in the second section, objective narrator is used--this narration is more like an objective report; and in the third section the narrator switches to third-person limited, in which the thoughts and feelings of a single character are the focus. Perhaps Bierce uses these three vantage points to present the complexities of the human mind.
The critic Cleanth Brooks and Robert Penn Warren have stated that Bierce's story depends too much on a quirk of human psychology and is thus a mere "case study" that does not reveal anything important about human nature, as good fiction does. Perhaps you could argue against this statement, using the various vantage points as an argument for how there is much that is revealed about the affects of war/the nature of war. Or, one can consider what is revealed with the 3 points of view about the psychology of a person in a life-or-death situation.
Aren't we splitting hairs here? Peyton might well be hallucination, but the author uses this as a flashback to earlier occurrences in Peyton's life. Hence it is a flashback. What I find interesting about the structure is that the story travels so far in such a short span of time for Peyton.
The story is structured in three parts, and each part is labeled, I, II, and III. Part I begins at Owl Creek Bridge as the company of soldiers is preparing to hang Peyton Farquhar. It ends with the sergeant stepping off the board on which Peyton stands, the rope around his neck.
Part II is exposition, the flashback that explains how Peyton got himself into so much trouble; he had been set up by a Federal scout (a spy) and had attempted to burn the bridge to stop the advancing Union army.
Part III takes the reader back to the bridge as Peyton falls through the timbers, the rope around his neck. At this point, Bierce performs a literary sleight-of-hand, making it seem that Peyton has escaped and managed to return home. At the end of Part III, however, Peyton hits the end of the rope, his neck breaks, and he dies, his body hanging below the bridge. The reader realizes that Peyton's "escape" had happened only in his mind in the few seconds between his fall and his death.
In studying the structure of the story, pay special attention to the last sentence in each part. Each one is significant in advancing the plot.
You clearly need to be aware of how the author uses flashback in the story to indicate what intense and powerful memories can be experienced by someone in the few seconds before they die. I don't want to give you a spoiler, but will assume that you have read the story and therefore realise that Peyton Farquhar doesn't escape - we are the privileged viewers of his imagined escape and outwitting of the troops, when in fact these imaginings are condensed into the time that it takes for a man to be pushed off a bridge with a noose around his neck.
One primary literary device or structure used in "The Occurrence at Owl Creek Bridge" is flashback. As he was preparing to be hanged, the man convicted of treason went back in time and we discover how he got where he is now. Once the hanging begins, the rope breaks. I assume you've read it, and you therefore understand that Peyton Farquhar never really left the site of his hanging but did "escape" in his mind. This is clearly a deception on the part of Ambrose Bierce; he manipulates time and space in order to deceive readers into believing Farquhar is actually free. Without these two elements, the story is one of a simple hanging for treason committed during the Civil War.
I wouldn't consider the story to be a flashback; it is more of a hallucination than anything else. The first part is where the he is about to be hanged and the sentry walks off the plank. The last part is where the rope snacks his neck. He is not having a flashback but is being hanged and thinking about returning to his wife.